This invention relates to a fluid seal, and more particularly, to a gasket adapted to be mounted between an engine head and block of an internal combustion engine, such as a diesel engine. In most automobile engines, and diesel engines, such as those used in power train engines, locomotives, trucks, road graders, tractors, electrical generators, marine engines and pumps, there exists a need for a gasket adapted to be mounted between the engine head and block. While some gaskets have been made out of fibrous materials, metal gaskets, and particularly steel gaskets, are particularly advantageous in certain environments. With the use of metal gaskets, a variety of problems have been confronted, among which is fretting.
Fretting, sometimes also referred to as fretting corrosion, is a serious problem and often causes failure of a seal. The phenomenon of fretting occurs in a gasket when it is placed between and in contact with the engine head and block, and is then subjected to a substantial compressive load or stress, together with high frequency and cyclic repetition rate of impulse forces and vibrations generated by the engine. The forces and vibrations tend to impart very slight relative movement between confronting metal surfaces such as between the gasket and the confronting block and head.
Fretting, resulting from the slight relative movement, causes surface pitting and deterioration to occur. This is usually accompanied by the formation of an oxide debris, which is reddish for steel. Fatigue cracks often start in the damaged area, although they may be obscured from observation by the surface debris. When the gasket is mounted in a diesel engine, such as in a truck, dust, abrasive particles and other road debris abrasively contact the gasket and contribute to fretting.
Fretting is caused by a combination of mechanical and chemical effects. Metal is removed from the surface either by a grinding action, or by alternative welding and tearing away of the high spots. The removed particles become oxidized and form an abrasive powder which continues the destructive process. Oxidation of the metal surface occurs and the oxide film is destroyed by the relative motion of the surfaces. When conditions are such that oxidation can occur, fretting damage is many times more severe.
Although fretting is frequently referred to as fretting corrosion, it is clear then that fretting is more related to wear resulting from grinding and the like then it is initially due to corrosion or fatigue. However, it differs from wear by the fact that the relative velocity of the contacting surfaces is much lower than is usually encountered in wear. Because the contacting surfaces are never brought out of contact, there is little or no chance for the corrosion products to be removed.
Although a number of efforts have been made to alleviate the problem of fretting corrosion in metal gaskets, the problem still remains a serious one in practice. For example, if all of the relative motion is prevented, then fretting will not occur. Increasing the forces normal to the surface may accomplish this. However the damage increases with the normal forces up to the point where relative motion is totally stopped and excessive forces are required to completely prevent relative movement. Exclusion of the atmosphere from the confronting surfaces will also reduce fretting, but this is frequently practically impossible to accomplish in automobile and diesel engines.